Since portable electronic devices, hybrid vehicles, etc., have had higher performance in recent years, secondary batteries (in particular, lithium primary batteries, lithium secondary batteries, and lithium ion second batteries) used for such devices are increasingly required to have higher capacity. However, for current lithium secondary batteries, the development of higher-capacity cathodes lags behind the development of higher-capacity negative electrodes. Even lithium nickel oxide-based materials, which are said to have a relatively high capacity, have a capacity of only about 190 to 220 mAh/g.
In contrast, sulfur, which has a theoretical capacity of as high as about 1,670 mAh/g, is a promising candidate for a high-capacity cathode material. However, elemental sulfur has low electronic conductivity, as well as problematic elution as lithium polysulfide into an organic electrolyte during charging and discharging. Therefore, a technique for inhibiting elution into an organic electrolyte is essential.
Metal sulfides are electronically conductive, and also have reduced elution into an organic electrolyte. However, metal sulfides have lower theoretical capacities than sulfur, and also have problematically low charge-discharge reversibility due to a great structural change resulting from lithium insertion/extraction during charging and discharging. To increase the capacity of metal sulfide, increasing sulfur content is necessary. However, since the sites of crystalline metal sulfide into which Li can be inserted during discharging are defined by crystal space groups, and this determines the maximum capacity, it is difficult to exceed this maximum capacity value.
For example, with respect to titanium sulfide compounds among metal sulfides, titanium disulfide (TiS2), titanium trisulfide (TiS3), etc., have been studied as crystalline titanium sulfides. Titanium disulfide and titanium trisulfide have been reported to have discharge capacities of about 240 mAh/g and about 350 mAh/g, respectively (Non-patent Literature (NFL) 1 and Non-patent literature (NPL) 2). However, a further increase of the capacity has been desired.
On the other hand, with respect to amorphous titanium sulfide compounds, a report describes that a TiSa (0.7≤a≤9) thin film was produced by using a pulsed layer deposition (PLD) method, and that an all-solid-state cell was charged or discharged (Non-patent Literature (NPL) 3). Further, other reports describe that when amorphous TiS3 or TiS4 was produced and used as an electrode in an all-solid-state cell, a high capacity (about 400 to 690 mAh/g) was obtained (Non-patent Literature (NPL) 4 and Non-patent Literature (NPL) 5).